I. Field of the Invention
The present invention relates to aircraft avionics systems in general, and more specifically to a method and electronic system for calibrating the Instrument Landing System portion of the aircraft avionics in order to reduce operational measurement errors which are inherent in the system.
II. Description of the Prior Art
Instrument landing systems provide path guidance to a runway by radiating a VHF localizer signal which defines the extended runway centerline and a UHF glideslope signal which defines a prescribed, obstacle-free glide path to a point on the runway. These two signals are modulated similarly by 90 Hz and 150 Hz audio tones. The signal-in-space characteristics are controlled such that, if an aircraft is on the defined centerline of either of the two components, the amplitudes of the detected 90 Hz and 150 Hz tones will be identical. As an aircraft deviates from the prescribed centerline, the amplitude of one or the other tone predominates. The difference in depth of modulation of the two tones (DDM) is proportional to the extent of deviation from the centerline. The ground transmitter facilities are arranged such that deviation to the right of a localizer course or below a glideslope results in a preponderance of 150 Hz tone.
To provide the pilot with uniformly sensitive deviation indications in different aircraft, world-wide standards have been adopted to relate tone amplitude differences to the deflection of typical flight path deviation indicators. For consistent and safe instrument approaches to a runway, both centering accuracy and deviation sensitivity of the airborne equipment are critical.
The avionics system in the typical fully-instrumented aircraft includes a VHF receiver for receiving localizer signals and a UHF receiver for receiving glide-slope signals. Each of these receivers is a separate operating entity and the present invention will utilize the signals from these receivers without substantial modification of the receivers themselves. While the present device is designed to be used with a new generation of VHF and UHF receivers, the invention could also be used with the Rockwell International/Collins Avionics GLS-350/350E Glode Slope Receiver and the Rockwell International/Collins Avionics VIR-351 Navigation receiver, among others.
In the conventional system illustrated in FIG. 1, similar electronic circuits are utilized to process the UHF glide slope and VHF localizer baseband output signals, both of which include 90 and 150 Hz signal components. The typical prior art system would process these signals in separate circuits in order to obtain d.c. output signals that are proportional to the deviation of the receiving location (the aircraft platform) from either the runway centerline or the prescribed descent path. The prior art typically utilizes active operational amplifier bandpass filters to separate the 90 and 150 Hz signal components, with the resulting output signals being rectified by a simple diode and resistor network. The output of the rectifier is then passed into the input of an operational amplifier low-pass filter in order to extract the d.c. component and to match impedance levels as required.
It is periodically necessary to align or pre-calibrate these prior art navigation systems by adjusting the relative levels between the 90 Hz and 150 Hz signals. The prior art systems accomplish this by utilizing a variable potentiometer between the two inputs to the bandpass filters in order to compensate for the differences in the gains of the bandpass filters, for differences in the turn-on voltages of the diodes, and for other operational parameters which will be different for different circuit elements chosen for the two signal paths.
It is also necessary to utilize a master gain (or deviation sensitivity adjust) control somewhere in the circuit in order to adjust the system sensitivity so that for a given voltage differential between the signals, a predetermined offset or deviation is produced in the position indicator or display for the pilot. It is extremely important that the electronic system operate in a linear fashion between the centered position and the known or calibrated offset positions so that the pilot will have an accurate indication of precisely how far his present position deviates from the desired flight path.
The problems with the prior art systems are primarily related to the number of adjustments which must be made and to the long-term stability of the components employed. The centering control and the deviation sensitivity control must be periodically adjusted in order to maintain the accuracy of the system. Also, it is difficult to obtain highly accurate performance specifications from mass-produced bandpass amplifiers due to the differences in circuit elements which are utilized therein, as well as changes caused by temperature, aging and other such factors. Any differential change in the performance of the system may not be immediately recognizable by the pilot because there is no readily available test signal to indicate when the deviation signal and/or the deviation indicator should be precisely at zero. Maintenence installations have test equipment for properly calibrating these adjustments, but this method of calibration is not only time consuming and inconvenient, but it is not useful when the pilot observes an error in high flight path readout while on a landing approach.
The present invention attempts to simultaneously calibrate both the 90 Hz and 150 Hz segments of the circuit by inserting a test signal during periodic pre-calibrations of the circuit, and then measuring the errors received in each leg of the circuit (both centering and gain variations), and then storing these measured errors in order to correct later measured results. Since the 90 Hz bandpass circuit is completely separate from the 150 Hz bandpass circuit, the absolute levels of the signals passing through each of the circuits must be critically controlled in order to obtain the amplitude error.
The applicants are unaware of any prior art references which are designed to operate in a fashion similar to the present invention. Harenberg, in U.S. Pat. No. 3,678,256, discloses a performance and failure assessment monitor which assesses overall performance of the automatic landing equipment in an aircraft flight control system. The monitor is connected to various sensors throughout the aircraft so that it can compare what the flight control system in the aircraft is accomplishing during a landing maneuver against an independent model generated within the monitor of what the flight control system should be accomplishing. The resulting comparison is displayed to the pilot as a measure of relative confidence that the landing will be accomplished properly. The disclosure of Harenberg relates to an analog-type system for modeling the function of the entire auto-pilot/landing system and does not relate specifically to eliminating distortion or other component errors in the system.